This invention relates to color television receiving apparatus and, more particularly, to improvements in such apparatus whereby the characteristics of a reproduced color video picture are controlled automatically in accordance with a transmitted reference signal or, in the absence of such a reference signal, in accordance with manual controls.
In a conventional NTSC composite color television signal, monochrome information, that is, the brightness information of the video picture, is transmitted by the luminance signal and the color information is transmitted by the chrominance signal. This color information is amplitude modulated on a subcarrier, and different color information components are modulated onto different phases of the subcarrier. The composite color video signal also includes periodic horizontal synchronizing signals and color burst signals which are used at the television receiver to control a local oscillator for chrominance signal demodulation such that the frequency and phase of the local oscillating signal are synchronized with that of the burst signal. For this purpose, the color burst signal is formed of approximately eight cycles of the color subcarrier which is superimposed onto the pedestal level during the horizontal synchronizing interval of the video signal. The composite video signal also includes vertical synchronizing information which is present during a number of horizontal line intervals that form the vertical retrace interval.
As aforementioned, the purpose of the color burst signal is to synchronize a local oscillator in a color television receiver so that the local oscillator output can be used to demodulate the received chrominance component with sufficient fidelity such that the reproduced color video picture is an accurate image of the original transmitted picture. However, a transmitted composite color video signal may be subjected to various phase and/or amplitude distortions. Such distortions may be caused directly at the transmitting site or may be attributed to the signal transmission path. Although the burst signal should be able to synchronize the local oscillator at the color television receiver to match or track such distortions, there is the possibility that the burst signal and the modulated color subcarriers may be subjected to different distortions. That is, since the burst signal is provided on the pedestal level and the color-modulated subcarrier is provided on quite different levels, the amplitude and phase distortion at the pedestal level might differ from such distortion at the other level. Consequently, even though this distortion in the burst level can be detected and corrected, such correction may not fully account for the distortions in the subcarrier at the other level. Similarly, transmission distortions may deleteriously affect the luminance component, resulting in noticeable defects in the reproduced video picture.
One proposal to minimize subcarrier distortion, which may not be fully corrected by synchronizing the local oscillator at the television receiver to the received burst signal, is to provide a further reference signal in the transmitted composite color video signal. This reference signal is known as the vertical interval reference signal, or VIR signal, and is formed of a reference chrominance component constituted by a subcarrier, equal in frequency and phase to the burst signal, transmitted during a selected portion of the vertical retrace interval. The VIR signal also includes a luminance reference component and a black level reference component. It has been suggested that this VIR signal can be transmitted during the nineteenth or twentieth horizontal line interval which, as is known, is included in the vertical retrace interval. Essentially, the VIR subcarrier reference signal differs from the burst signal in that it exists for a longer duration and is superimposed on a level other than the pedestal level.
It is expected that, when a transmitted video signal having the VIR signal is received, the various reference information provided by the VIR signal can be used to control the color television receiving apparatus such that the reproduced color picture exhibits optimum color characteristics. That is, this VIR signal can be used to control the phase of the generated local oscillator which, as is known, determines the hue or tint of the color picture. Also, the information provided by this VIR signal can be used to control the level of the received chrominance component which, as is known, controls the saturation of the reproduced color picture. Therefore, by automatically controlling the local oscillator phase and chrominance component level in accordance with optimum standards, such as the NTSC standard, as represented by the VIR signal, a corresponding optimum color video picture can be reproduced. One type of color control circuitry is described in copending application 753,868, filed on Dec. 23, 1976 by the inventors of the instant invention.
In the aforementioned application, suitable control apparatus is provided to permit the color characteristics of the video picture to be controlled in accordance with a viewer's preferences, whether or not the transmitted composite color television signal includes the VIR signal. It also is desirable to utilize the luminance and black level information in the VIR signal to control the luminance level of the reproduced video picture; and such luminance level control should be subject to a viewer's subjective preferences in the absence of the VIR signal.